This invention pertains to methods and apparatus for determining the state of charge of a secondary battery for the purpose of adjusting the charging rate thereof. More specifically, the invention pertains to methods and apparatus for determining the "gassing point", that is, that charge state at which the electrolyte of the cell begins to break down into gas bubbles.
It has long been known that when attempting to fast-charge secondary cells with fluid electrolytes, such as the common lead-acid storage battery, the cell is capable of accepting high charging currents only until approximately 80-90% charged. At that point the additional energy put into the battery is largely used to break down the electrolyte into hydrogen and oxygen gas, rather than being stored as chemical energy. The charging rate must then be reduced, lest damage occur to the cell through overheating, "boiling" away of the water part of the electrolyte (with a resulting harmful increase in acid concentration), or corrosion of the plates through excessive gassing causing a loosening of the active material.
Many methods have been used to detect this "gassing point" indirectly. Most common are those which sense cell voltage and/or charging current (see, i.e. Van Beek, U.S. Pat. Nos. 4,217,533; Hess, 4,207,513; Scott 4,152,635; Lavell 4,052,656; Clayton 3,660,748; and others). Other indirect methods include use of a timed charger (Hase 3,848,173; Saslow 3,178,629; Buckholder 2,670,039), temperature detectors (Brown 4,061,956; Guelpa 2,666,883), or watt-hour or Coulometers (Sarbacher 3,889,170; Wilson 3,421,067; and many others). All of these indirect methods will, through their indirect nature, tend to approximate the proper charge rate at best. The actual gassing point will vary in response to battery temperature especially, but also to electrolyte condition, age and composition of the cell, and many other factors. To even approximate the proper charge rate an indirect sensing charger must be matched to a specific battery, and even then cannot directly and reliably compensate for environment and age factors.
In some charger designs the charge rate is tied to the specific gravity of the electrolyte by placing a hygrometer in the electrolyte and having the movement of the float interrupt a photoelectric control light beam (Knight U.S. Pat. No. 2,310,700 and others). This method has the same drawbacks as the other indirect methods, and also requires modification of the cell and addition of a potentially fragile external test column.
Direct gassing detectors have most commonly tended to respond to gas pressure in the cell (i.e. Ebbert U.S. Pat. Nos. 3,652,915; Hanson 2,118,558; and others). These have tended to be less than completely successful due to the effects of battery cell volume, temperature, and the small volume of gas produced.
Other more exotic schemes involving recombination of water on a catalytic agent (Zollner, German Pat. No. 28.08.76-DT-638899), injection of ionized gasses (Genin U.S. Pat. Nos. 2,514,235), changes in the velocity of sound (Molyneux 3,798,528), or addition of a third electrode to the battery (Duddy 3,901,729 and current production Yuasa batteries) have been tried as well. All of these systems require sealing and/or otherwise modifying the cell, or addition of complicated, possibly fragile, apparatus to the battery, often involving substances which must be replaced periodically.
Thus, it is an object of this invention to provide an improved method for detecting bubbling on the surface of a liquid, for example the bubbling characteristic of the "gassing point" of a battery under charge.
It is a further object of the invention to provide an improved battery gassing detector for use with secondary battery charging circuits.
Further, it is an object of this invention to provide a simple, rugged, means of directly detecting when a cell has reached the gassing point which can be used with any liquid electrolyte battery without sealing or modifying the battery, and without externally handling the electrolyte or gas.
The direct photoelectric detection of bubbling on the surface of a liquid represents a novel and distinct advance in the art. In battery charger applications, it represents an advance over time, voltage, and other indirect methods because it detects gassing directly, thus the effects of variables such as pressure, temperature and battery condition become unimportant. The photoelectric detector disclosed by this application represents an advance over any of the methods requiring modification of the cell because it can be employed by the designer in a charging system for any liquid-electrolyte battery without the expense and complication of requiring the battery manufacturer to supply modified cells. Moreover, it can be used with existing cells with no retrofitting of additional electrodes, etc. Unlike the catalytic and third-electrode systems, the detector is not consumed and needs no more maintenance than a simple cleaning when checking electrolyte level. It represents an advance over any of the systems which pipe electrolyte or gas outside the battery case in that it requires no external pipes or attachments which are a source of problems in the rugged environment of a motor vehicle.
Other advantages will become apparent upon further examination of the disclosure.